Variable resistor construction



D 23, 1952 A R. w. sEARLE ET A1. 2,623,144

ARIABLE REsIsToR CONSTRUCTION I -Filed April 20, 1951 JOI Patented Dec.23, i952 OFFICE VARIABLE RESISTOR CONSTRUCTION Raymond W. Searle,Waltham, and Lauren M.

Burgess, Lexington, Mass., assignors to Technology InstrumentCorporation, Acton, Mass.

Application April 20, 1951, Serial No. 222,016

(C1. Ztl-48) 4 Claims.

The present invention relates to an improved variable resistorconstruction and the method of assembly thereof.

In the manufacture of precision variable resistors, generally calledpotentiometers, particularly suited for servo mechanisms, it isnecessary to provide a construction which has stable characteristics notaiiected by changes in temperature and humidity. T.n the construction ofsuch variable resistors substantially all parts thereof are coated withvarnish and baked. Due to the temperature of the baking process certainprecautions Inust be taken to insure retaining the resistance element inposition.

It therefore is an object of the present invention to provide animproved variable resistor construction and method ci assembly.

A still further object of the present invention is to provide animproved variable resistor having means for retaining in position thevariable resistance element during its manufacture and throughout theuseful life of the resistor.

Other and further objects of the present -invention subsequently willbecome apparent by reference to the following description taken inconjunction with the accompanying drawing; wherein Figure l is an endview of a variable resistor,

with the cover removed, constructed in accordance with the presentinvention;

Figure 2 is a cross-sectional View oi a variable resistor shown inFigure 1 ras seen in the direction of the arrows along the line 2 2; and

Figure 3 is a perspective view of the retaining ring element employed inthe resistor of Figures l and 2.

Referring to the drawing, the resistor construction illustrated employsa cylindrical casing II having a cylindrical Wall i2 and an end wail i3.The end wall i3 is provided with a boss ifi for supporting a bearing i5for a shaft I6. The bearing I5 has a threaded end Il for receiving amounting nut for holding the variable resistor in position on anapparatus. rlhe shaft it is provided vvith a groove into which is ntteda, split ring IS which engages the end of the bearing member IE. Theshaft i@ at its other end carries a split sleeve I9 which is engaged byan arm 2l. The arm 2| at its lower end is provided with biiurcatedportions 22, which may be drawn together by a stud bolt 23 passingthrough a suitable aperture in one of the bifuroated portions andengaging a threaded aperture in the other bifurcated portion 22. At itsouter end the arm 2I has a threaded opening for receiving a screw 24which engages a contact assembly Eil having an outwardly extending arm25, which carries a precious alloy contact element 2l'. The preciousalloy contact element 2l engages the turns of resistance wire on aresistance element. The contact assembly 2t also includes two similarbrush arms 28, each provided with a precious alloy contact 2Q whichengages a slip ring 3l. The slip ring ill is mounted on an insulatedbushing 32, which'ts over the bearing boss ifi of the end wall i3 andthe end portion of the bearing sleeve l5. A slip ring iii is suitablyconnected to the center terminal 32 of a plurality ci terminals carriedby a terminal block 32 mounted on the outside of thecasing l I.

Mounted within the casing i l adjacent the inside of the cylindricalwall l2 is a resistance element 3d formed of a ilat strip of insulatingmaterial about which has been wound in spaced relation a resistance wireThe ends of the resistance wire 5 are connected to electrical terminals3e and Eil carried by the terminal block 33. In order to assureretaining the resistance element 3d in position during the assembly andduring the useful life of the variable resistor, there is provided aretaining ring 38, which is shown in perspective in Figure S.

This split retaining ring 35% is positioned against the end wall i3 ofthe housing and against the resistance element til. The split ring 33 isexpanded by a wedge t9 which engages the split of the ring, and which isprovided with a central aperture l. The central aperture il receives ascrew d2 which engages a threaded opening i3 in the end wall I3 of thehousing H. The housing II is provided with a cover ed, which is providedwith a vent or aperture i.

In order to assemble the device heretofore cle-- scribed so as toprovide stable characteristics not affected materially by variations intemperature and humidity, the following method of manufacture isemployed. All insulating material elements, such as the resistanceelement strip it, the insulated sleeve 32, and the retaining ring 32,together with its wedge Iii?, are baked for about one-half hour in anoven at appr Xiinately 212 lf". to drive ofi all possible moisture.lThereafter all exposed surfaces of these insulated elements arevarnished. The entire inside of the housing I I is also varnished. Theseelements are then baked in a well ventilated oven for several hours at atemperature of 275 F.

Thereupon the slip ring bushing s2 is inserted into the varnished base,the slip ring 3l is placed in position, then insulating members areinserted to line the inside cylindrical Wall I2 and the back end wallI3. These insulating members are varnished into position and a suitablevarnish is employed. The varnish is then applied to the exposed edges ofthe slip ring insulating bushing -both above and below the slip ringwith a slight overlap of the varnish into the slip ring. This is thenbaked for several hours at a temperature of 275 F.

Previous to assembling other elements of the variable resistor, theresistance wire supporting strip 34 has been varnished and baked.Resistance Wire is applied thereto and suitably secured in position. Thebrush arm assembly is mounted on a dummy shaft and all exposed surfacesof the assembly are carefully painted with varnish, except for theprecious metal contacts 2'I and 29. This is also baked at a temperatureof 275 F. for approximately two hours. At the same time the inside ofthe cover 44 is varnished and likewise baked for a similar time at thesame temperature. The terminals 32, 36 and 31 are pressed into themounting plate 33 and varnished into position.

Thereafter, the resistance element strip 34 is varnished with exceptionof a buffed track for the brush arm. While the varnish is still wet, theresistance element is inserted into the housing II. The ring 38 and itsWedge 39 are again Varnished and then :placed into position within thecasing II. Additional varnish is applied at the place where the ring 38engages the resistance element 34. The terminal plate 33 is coated withthe varnish and mounted in position. Varnish is now applied to theoutside of the casing II, including the bushing I up to the point wherethe threads II begin. This is then baked for several hours at atemperature of 275 F.

Next, electrical connections are completed to the terminals of theresistance Wire 35 and to the slip ring 3l. The brush arm assembly isthen installed on the end of the shaft I6. The exposed end of the shaftis varnished and varnish is extended over the brush arm 2l. Varnish isapplied at the fitting edges on both the cover and the .base whereuponthe cover is pressed into position. The entire outside surface of thecover and the edges of the housing II are varnished. This is then again:baked at a temperature of 275 F. for approximately two hours.

To illustrate the application of varnish to the various surfaces, doublelines have been used in Figures 1 and 2.

While for the purpose of illustrating and describing the presentinvention a particular embodiment has been shown in the drawing, it isto be understood that the invention is not to be limited thereby, sincesuch variations in construction and in assembly are contemplated as maybe commensurate with the spirit and scope of the invention defined inthe accompanying claims.

We claim as our invention:

1. In a variable resistor the combination comprising a cylindricalhousing having a closed end provided with a bearing support for a shaft,a resistance element formed of a iiat strip of insulation wound withspaced resistance Wire, said element being located against the insidecylindrical wall of said casing, a split ring of insulation mountedagainst said housing end and against said resistance element, and meansengaging the split of said ring for expanding said ring to retain saidresistance element against the inside of the cylindrical Wall of saidcasing.

2. A variable resistor construction comprising a cylindrical housinghaving an end wall, a resistance element formed of a flat strip ofinsulation Wound with spaced resistance wire, said element being locatedagainst the inside cylindrical wall of said casing, a split ring ofinsulation mounted against, said housing end and against said resistanceelement, and means for expanding said ring to retain said resistanceelement against the inside cylindrical wall of said casing comprising awedge engaging the split in said ring.

3. The variable resistor construction comprising a cylindrical housinghaving an end Wall containing a threaded aperture, a resistance elementformed of a flat strip of insulation material wound with spacedresistance Wire, said element being positioned against the insidecylindrical wall of said casing, a split ring of insulation materialmounted against said housing end and against said resistance element,and means for expanding said ring to retain said resistance element inposition comprising a wedge engaging the split in said ring, and a screwpassing through said wedge and the threaded aperture of said end Wall.

4. The method of manufacturing a variable resistor having a cylindricalhousing, a cover, a resistance element mounted on an insulated strip, abrush assembly therefor, and an insulated retaining ring comprising thesteps of baking said strip and said ring for about one-half hour at atemperature of at least 212 F., coating with varnish said resistanceelement, said retaining ring, said brush arm assembly and the inside ofsaid housing and said cover, baking said coated elements for severalhours at a temperature `of approximately 275 F., varnishing the insidecylindrical wall of said housing, inserting said resistance elementagainst said wall, varnishing said retaining `ring and inserting itagainst said resistance element, baking said assembled housing,resistance element and retaining ring for several hours at approximately275 F., inserting said brush assembly, varnishing the edges of saidcover, applying said cover, varnishing the outside of said housing andcover, and baking the assembled variable resistor for several hours atapproximately 275o F.

RAYMOND W. SEARLE. LAUREN M. BURGESS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,876,921 Hall 1 Sept. 13, 19322,015,425I Gaubert Sept. 24, 1935 2,476,294 Hampton July 19, 1949

